Beam shaping and projection imaging with solid state UV gaussian beam to form vias

ABSTRACT

A diode-pumped, solid-state laser ( 52 ) of a laser system ( 50 ) provides ultraviolet Gaussian output ( 54 ) that is converted by a diffractive optical element ( 90 ) into shaped output ( 94 ) having a uniform irradiance profile. A high percentage of the shaped output ( 94 ) is focused through an aperture of a mask ( 98 ) to provide imaged to provide imaged shaped output ( 118 ). The laser system ( 50 ) facilitates a method for increasing the throughput of a via drilling process over that available with an analogous clipped Gaussian laser system. This method is particularly advantageous for drilling blind vias ( 20   b ) that have better edge, bottom, and taper qualities than those produced by a clipped Gaussian laser system. An alternative laser system ( 150 ) employs a pair of beam diverting galvanometer mirrors ( 152, 154 ) that directs the Gaussian output around a shaped imaging system ( 70 ) that includes a diffractive optical element ( 90 ) and a mask ( 98 ). Laser system ( 150 ) provides a user with the option of using either a Gaussian output or an imaged shaped output ( 118 ).

RELATED APPLICATIONS

This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 10/188,282, filed Jul. 1, 2002, now abandoned whichis a continuation-in-part of U.S. patent application Ser. No.09/580,396, filed May 26, 2000, now U.S. Pat. No. 6,433,301 whichderives priority from U.S. Provisional Application No. 60/193,668, filedMar. 31, 2000, from U.S. Provisional Application No. 60/175,098, filedJan. 7, 2000, and from U.S. Provisional Application No. 60/136,568,filed May 28, 1999.

COPYRIGHT NOTICE

© 2001 Electro Scientific Industries, Inc. A portion of the disclosureof this patent document contains material which is subject to copyrightprotection. The copyright owner has no objection to the facsimilereproduction by anyone of the patent document or the patent disclosure,as it appears in the Patent and Trademark Office patent file or records,but otherwise reserves all copyright rights whatsoever. 37 CFR §1.71(d).

TECHNICAL FIELD

The invention relates to a diode-pumped solid-state laser and, inparticular, to employing such a laser to generate an ultraviolet laserbeam having a TEM₀₀ non-astigmatic spatial mode to drill vias.

BACKGROUND OF THE INVENTION

U.S. Pat. Nos. 5,593,606 and 5,841,099 of Owen et al. describetechniques and advantages for employing UV laser systems to generatelaser output pulses within advantageous parameters to form through-holeor blind vias through at least two different types of layers inmultilayer devices. These parameters generally include nonexcimer outputpulses having temporal pulse widths of shorter than 100 ns, spot areaswith spot diameters of less than 100 μm, and average intensities orirradiances of greater than 100 mW over the spot areas at repetitionrates of greater than 200 Hz.

Lasers are described herein only by way of example to ultraviolet (UV)diode-pumped (DP) solid-state (SS) TEM₀₀ lasers that generate a naturalGaussian irradiance profile 10 such as shown in FIG. 1, but thedescription is germane to almost any laser generating Gaussian output.Ablating particular materials with any laser, and particularly a UV DPSSlaser, is contingent upon delivering to a work piece a fluence or energydensity (typically measured in units of J/cm²) above the ablationthreshold of the target material. The laser spot of a raw Gaussian beamcan be made quite small (typically on the order of 10 to 15 μm at the1/e² diameter points) by focusing it with an objective lens.Consequently the fluence of the small focused spot easily exceeds theablation threshold for common electronic packaging materials,particularly the copper typically used in the metallic conductor layers.Hence, the UV DPSS laser, when used in a raw, focused beamconfiguration, is an excellent solution for drilling vias through one ormore copper layers in an electronic packaging work piece. Since thefocused spot is typically smaller than the desired size of the via, thefocused spot is moved in a spiral, concentric circular, or “trepan”pattern to remove sufficient material to obtain the desired via size.This approach is commonly referred to as spiraling or trepanning withthe raw, focused beam. Spiraling, trepanning, and concentric circleprocessing may generically be referred to as “nonpunching” forconvenience.

An alternative approach that is also well known in the art involvespassing the TEM₀₀ laser beam with the Gaussian irradiance profilethrough a circular aperture or mask of a predetermined diameter 12. Oneor more common refractive optic lenses are then used to project an imageof the illuminated aperture onto the work surface. The size of theimaged circular spot depends on both the size of the aperture andoptical de-magnification obtained with the refractive imaging lens orlenses. This technique, known as projection imaging or simply imaging,obtains a desired via diameter by adjusting either the aperture size orthe optical de-magnification or both, until the size of the imaged spotmatches the desired via size. Because the low-intensity “wings” of theGaussian irradiance profile are masked or clipped by the aperture edges,this imaging technique is, therefore, also called clipped-Gaussianimaging.

When drilling vias with the imaged spot, the laser beam simply dwells atthe via site for a number of pulses until sufficient material has beenremoved. This drilling method, often called “punching,” eliminates theextremely precise and fast in-via movement of the laser spot that isrequired when trepanning or spiraling with the raw, focused beam. Thus,via drilling with a clipped Gaussian beam reduces the demands placedupon the high-speed beam positioner, since it eliminates the complexsmall-radius, curved pathways and attendant high accelerationsassociated with inside-the-via motions. Process development is alsosimpler with projection imaging because there are fewer processparameters to be optimized.

Clipped Gaussian processing also produces much rounder and morerepeatable vias because the inherent variations in laser spot roundnessfrom laser to laser no longer govern the shape of the via, rather theroundness is largely determined by the circularity of the aperture andthe quality of the optics used to project its image onto the worksurface. Roundness is also secondarily impacted by throughput and thedegree to which the wings of the raw Gaussian pulse is clipped.Roundness, or circularity, may be quantified as a ratio of minimumdiameter to the maximum diameter typically measured at the top of thevia, i.e. R=d_(min)/d_(max). The rounder spots are possible because onlythe central portion of the Gaussian irradiance profile of the laser beamis permitted to pass through the aperture; hence the low-irradianceouter regions of the Gaussian beam are blocked or clipped by theaperture mask.

A problem with a clipped Gaussian beam is, however, that its center ismore brightly illuminated than its edges. This nonuniformity adverselyaffects the quality of vias created with this beam, particularly blindvias, resulting in vias having rounded bottoms and uneven edges andrisking damage to the underlying or neighboring substrate.

A laser system employing the clipped Gaussian technique can beimplemented so that varying fractions of the Gaussian beam are blockedby the aperture. If the Gaussian irradiance profile is highly clipped sothat only a small portion of the output beam center is allowed to passthrough the aperture, then the irradiance profile imaged onto the worksurface will be more nearly uniform. This uniformity comes at theexpense of rejecting a large fraction of the energy at the aperture maskand hence not delivering the energy to the work surface. Wasting suchlarge portions of beam energy impedes drilling speed.

If, on the other hand, a large fraction of the beam energy is permittedto pass through the aperture, then higher fluence is delivered to thework. However, the difference between the irradiance at the spot center,I_(c), and the spot edges, I_(e), will be large. The fraction of energypassing through the aperture is commonly known as the transmissionlevel, T. For a Gaussian beam, the following mathematical relationshipexists:

T=1−I _(e) /I _(c)

For example, if 70% of the beam energy passes through the aperture, thenboth the irradiance and the fluence at the edge of the imaged spot willbe only 30% of the value at the center of the spot. This differencebetween I_(c) and I_(e) causes tradeoffs in the drilling process.

If high laser power is used in order to drill more rapidly, the fluenceat the spot center, Fc, can exceed the fluence at which the copper atthe via bottom begins to melt and reflow. At the same time, if T islarge (and therefore the edge-to-center fluence ratio Fe/Fc within thespot is small), the edges of the imaged spot have low fluence and do notablate the organic dielectric material rapidly. FIG. 2 is graph of edgefluence versus aperture diameter for clipped Gaussian output undertypical via processing parameters. As a result, many pulses are requiredto clear the dielectric material (such as an epoxy resin) from the edgesof the via bottom and thereby obtain the desired diameter at the viabottom. Applying these pulses, however, may damage the center of the viadue to the high fluence in that region which melts the bottom copper.

The clipped Gaussian technique, therefore, forces a trade-off betweenhigh pulse energy that drills rapidly but damages the center of the viabottom and lower pulse energy that is below the copper reflow thresholdfluence but drills slowly and requires many pulses to clear the viaedges. Typically, depending on the via size, transmission levels between30% and 60% offer an acceptable compromise between wasted (blocked)laser energy and the undesirable process phenomena related tonon-uniformity of the fluence within the imaged spot. Small vias can bedrilled at acceptable speed with lower transmission levels (e.g. 25-30%)and therefore higher uniformity of the imaged spot. However, for manyapplications, 50%<T<60% is desirable to obtain acceptable speed, and viaquality suffers due to bottom copper damage.

A more energy and speed efficient method for drilling vias is thereforedesirable.

SUMMARY OF THE INVENTION

An object of the present invention is, therefore, to provide a methodand/or system that improves the speed or efficiency of via drilling witha Gaussian beam while improving the via quality.

Another object of the invention is to provide such a method or systemthat employs a UV, diode-pumped (DP), solid-state (SS) laser.

The present invention enhances the projection imaging technique. In oneembodiment of the invention, a UV DPSS laser system is equipped with adiffractive optical element (DOE) to shape the raw laser Gaussianirradiance profile into a “top hat” or predominantly substantiallyuniform irradiance profile. The resulting shaped laser output is thenclipped by an aperture or mask and focused by a scan lens to provide animaged shaped output beam at a target surface. The imaged shaped outputbeam has a substantially uniform intensity laser spot from its center toits edge so that high quality vias can be drilled rapidly without riskof bottom damage.

Conventional systems that utilize beam shaping, projection imaging, ordiffractive optics employ low brightness non-UV lasers or highlyastigmatic and multi-mode Excimer lasers and have been generally used inapplications other than materials processing.

In many of these via drilling applications, the spatial uniformity isrequired to make the process work. Without it, the non-uniformity of thefluence at the work surfaces typically leads to problems withover-processing in the center of the focused spot and under-processingat its edges. In the present invention, the beam shaping does not enablethe via drilling process. Rather, it enhances it by making the processfaster and more controllable. The invention therefore provides theability to enhance the quality, speed, and robustness of the UV laservia drilling process.

Although other types of devices have been used to produce near-uniformor “homogenized” beams with excimer lasers for materials processing,such homogenizers do not work with the highly coherent, near-TEM₀₀spatial mode of a DPSS high-brightness laser. Further, since unlike thelarge spots inherent to an excimer laser beam, the TEM₀₀ spatial mode ishighly focusable, so the present invention can utilize a much higherpercentage of the incident energy.

Additional aspects and advantages of this invention will be apparentfrom the following detailed description of preferred embodimentsthereof, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a three-dimensional Gaussian irradianceprofile of a typical prior art DPSS laser pulse.

FIG. 2 is graph of edge fluence versus aperture diameter for clippedGaussian output under typical via processing parameters.

FIG. 3 is an enlarged, cross-sectional side view of vias drilled into aportion of a generic laser work piece.

FIG. 4A is a simplified side elevation and partly schematic view of anembodiment of a laser system employed for increasing via drillingthroughput in accordance with the present invention.

FIG. 4B is a simplified side elevation and partly schematic view of analternative embodiment of a laser system employed for increasing viadrilling throughput in accordance with the present invention.

FIGS. 5A-5C is a sequence of simplified irradiance profiles of a laserbeam as it changes through various system components of the laser systemof FIG. 4.

FIGS. 6A-6D are exemplary substantially uniform square or circularirradiance profiles.

FIGS. 7A-7D are simplified side elevation and partly schematic views offour respective exemplary embodiments of beam shaping systems forvarying the size of an image spot.

FIG. 8 is a simplified partly schematic plan view of alternative lasersystem that employs an auxiliary galvanometer mirror pathway to permituse of a raw focused beam.

FIG. 9 is a graphical comparison of ideal fluence distributions at theaperture plane for imaged shaped output and clipped Gaussian output atseveral typical transmission levels under typical via processingparameters.

FIG. 10 is a graphical comparison of throughput curves for clippedGaussian and imaged shaped via drilling techniques.

FIG. 11 is a graph of via taper ratio as a function of work surfacelocation relative to the nominal image plane.

FIG. 12 is a graph of via diameter as a function of work surfacelocation relative to the nominal image plane.

FIG. 13 is a graph of via roundness as a function of work surfacelocation relative to the nominal image plane.

FIG. 14 is a copy of an electron micrograph of a 75-μm via drilled in45-μm thick epoxy resin.

FIG. 15 is a copy of an electron micrograph a 75-μm via drilled in 45-μmthick epoxy resin in a 150-μm thick pre-etched copper opening.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 3 is an enlarged, cross-sectional side view of though hole via 20 aand blind via 20 b (generically via 20) machined into a generic laserwork piece 22 that may, for example, be an MCM, circuit board, orsemiconductor microcircuit package. For convenience, work piece 22 isdepicted as having only four layers 24, 26, 28, and 30.

Layers 24 and 28 may contain, for example, standard metals such as,aluminum, copper, gold, molybdenum, nickel, palladium, platinum, silver,titanium, tungsten, metal nitrides, or combinations thereof.Conventional metal layers 24 and 28 vary in thickness, which istypically between 9-36 μm, but they may be thinner or as thick as 72 μm.

Layer 26 may, for example, contain a standard organic dielectricmaterial such as benzocyclobutane (BCB), bismaleimide triazine (BT),cardboard, cyanate esters, epoxies, phenolics, polyimides,polytetrafluorethylene (PTFE), various polymer alloys, or combinationsthereof. Conventional organic dielectric layers 26 vary considerably inthickness, but are typically much thicker than metal layers 24 and 28.An exemplary thickness range for organic dielectric layers 26 is about30-400 μm, but they may be placed in stacks as large as 1.6 mm.

Layer 26 may also contain a standard reinforcement component or “layer”30. Layer 30 may be fiber matte or dispersed particles of, for example,aramid fibers, ceramics, or glass woven or dispersed into organicdielectric layer 26. Conventional reinforcement layers 30 are typicallymuch thinner than organic dielectric layers 26 and may be on the orderof 1-2 μm and perhaps up to 10 μm. Skilled persons will appreciate thatreinforcement material may be introduced as powders into the organicdielectrics. The layers 30 formed by such powdery reinforcement materialmay be noncontiguous and nonuniform. Skilled persons will alsoappreciate that layers 24, 26, and 28 may also be internallynoncontiguous, nonuniform, and nonlevel. Stacks having several layers ofmetal, dielectric, and reinforcement material may be larger than 2 mm.

A through-hole via 20 a typically penetrates all layers and materials ofwork piece 22 from its top 42 to its bottom 44. Blind via 20 b does notpenetrate all layers and/or materials. In FIG. 3 for example, blind via20 b stops at and does not penetrate layer 28. The taper of a via 20 iscommonly discussed in terms of a ratio of its bottom diameter d_(b) toits top diameter d_(t). A taper ratio of 66% is currently an acceptablestandard in the industry, and ratios of 67-75% are considered to be verygood. The present invention permits a taper ratio of greater than 80% ata maximum throughput with no damage to layer 28, and taper ratios ofgreater than 95% are possible without damage to layer 28.

Via diameters typically range from 25-300 μm, but laser system 50 (FIG.4) may produce vias 20 a and 20 b that are as small as about 5-25 μm orgreater than 1 mm. Vias smaller than 150 μm diameter are preferablyproduced by laser punching. Vias larger than 180 μm are preferablyproduced by trepanning, concentric circle processing, or spiralprocessing.

With reference to FIG. 4A, a preferred embodiment of a laser system 50of the present invention includes Q-switched, diode-pumped (DP),solid-state (SS) UV laser 52 that preferably includes a solid-statelasant such as Nd:YAG, Nd:YLF, Nd:YAP, or Nd:YVO₄, or a YAG crystaldoped with holmium or erbium. Laser 52 preferably provides harmonicallygenerated UV laser pulses or output 54 at a wavelength such as 355 nm(frequency tripled Nd:YAG), 266 nm (frequency quadrupled Nd:YAG), or 213nm (frequency quintupled Nd:YAG) with primarily a TEM₀₀ spatial modeprofile.

Although Gaussian is used to describe the irradiance profile of laseroutput 54, skilled persons will appreciate that most lasers 52 do notemit perfect Gaussian output 54 having a value of M²=1. For convenience,the term Gaussian is used herein to include profiles where M² is lessthan or equal to about 1.4, even though M² values of less than 1.3 or1.2 are preferred. Skilled persons will appreciate that otherwavelengths are available from the other listed lasants. Laser cavityarrangements, harmonic generation, and Q-switch operation are all wellknown to persons skilled in the art. Details of one exemplary laser 52are described in detail in U.S. Pat. No. 5,593,606 of Owen et al.

UV laser pulses 54 may be converted to expanded collimated pulses orbeam output 60 by a variety of well-known optics including beam expanderor upcollimator lens components 56 and 58 (with, for example, a 2× beamexpansion factor) that are positioned along beam path 64. Beam output 60is directed through a shaping and imaging system 70 to producecollimated apertured shaped pulses or beam output 72 that is thenpreferably directed by a beam positioning system 74 to target collimatedapertured shaped output 72 through a scan lens 80 (The scan lens is alsoreferred to as a “second imaging,” focusing, cutting, or objectivelens.) to a desired laser target position 82 at the image plane on workpiece 22.

Beam positioning system 74 preferably includes a translation stagepositioner 76 and a fast positioner 78. Translation stage positioner 76employs at least two platforms or stages that supports, for example, X,Y, and Z positioning mirrors and permits quick movement between targetpositions 82 on the same or different circuit boards or chip packages.In a preferred embodiment, translation stage positioner 76 is asplit-axis system where a Y stage supports and moves work piece 22, an Xstage supports and moves fast positioner 78 and objective lens 80, the Zdimension between the X and Y stages is adjustable, and fold mirrors 75align the beam path 64 through any turns between laser 52 and fastpositioner 78. Fast positioner 78 may for example include a pair ofgalvanometer mirrors that can effect unique or duplicative processingoperations based on provided test or design data. These positioners canbe moved independently or coordinated to move together in response topanelized or unpanelized data. Such a preferred beam positioning system74 that can be used for drilling vias 20 is described in detail in U.S.Pat. No. 5,751,585 of Cutler et al.

A laser controller (not shown) that directs the movement of the beampositioning components preferably synchronizes the firing of laser 52 tothe motion of the components of beam positioning system 74 such asdescribed in U.S. Pat. No. 5,453,594 of Konecny for Radiation BeamPosition and Emission Coordination System.

An example of a preferred laser system 50 that contains many of theabove-described system components employs a Model 45xx UV laser (355 nm)in a Model 5200 laser system or others in its series manufactured byElectro Scientific Industries, Inc. in Portland, Oreg. Persons skilledin the art will appreciate, however, that any other laser type having aGaussian beam intensity profile, other wavelengths such as IR, or otherbeam expansion factors can be employed.

FIG. 4B is a simplified side elevation and partly schematic view of analternative embodiment of laser system 50 employing a variable beamexpander 55 up stream of imaging system 70 and an optional variable zoombeam expander 120 a (later discussed with respect to FIG. 7) that ispositioned downstream of the IOR. For convenience, certain features oflaser system 50 in FIG. 4B that correspond to those in FIG. 4A have beendesignated with the same reference numbers. FIGS. 4A and 4B maygenerically be referred to as FIG. 4.

With reference to FIG. 4B, variable beam expander (VBE) 55 includesaxially movable expansion and collimation lens components 57 and 58whose positions can be adjusted to obtain a beam of desired size at beamshaping component 90. Because the irradiance profile 96 at aperture mask98 depends the diameter of the Gaussian-profile input beam 60 at beamshaping component 90 (discussed in detail in connection with FIG. 5),lens components 57 and 58 can be adjusted to better control profile 96at the aperture mask 98. Moreover, VBE 55 permits a higher opticalthroughput for all sizes of aperture mask 98 for the following reason.The diameter of the shaped output 94 at the plane of aperture mask 98can be optimized by varying the distance Z so that most of the beampasses through aperture 98, yielding high optical throughput. However,at the same time, this creates undesirable variations in the irradianceprofile 96. VBE 55 compensates for these variations in profile 96created by varying distance Z. So used in this fashion, VBE 55 permitsoptical throughput at the aperture to be maximized while avoidingundesirable effects of the procedure used to do so. VBE 55 also permitscompensation for variations in beam shaping components 90 and/or forvariations in beam size from laser to laser, and permits greaterconsistency between several laser systems 50. The axial positions oflens components 57 and 58 can be manually or automatically adjusted withthe aid of the laser controller.

FIGS. 5A-5C (collectively FIG. 5) show a sequence of simplifiedirradiance profiles 92, 96, and 102 of a laser beam as it changesthrough various system components of laser system 50. FIGS. 5Ba-5Bc showsimplified irradiance profiles 96 a-96 c of shaped output 94 (94 a, 94b, and 94 c, respectively) as a function of distance Z with respect toZ₀′. Z₀′ is the distance where shaped output 94 has its flattestirradiance profile shown in irradiance profile 96 b.

With reference again to FIGS. 4 and 5, a preferred embodiment of shapedimaging system 70 includes one or more beam shaping components 90 thatconvert collimated pulses 60 that have a raw Gaussian irradiance profile92 into shaped (and focused) pulses or output 94 b that have anear-uniform “top hat” profile 96 b, or particularly a super-Gaussianirradiance profile, in proximity to an aperture mask 98 downstream ofbeam shaping component 90. FIG. 5Ba shows an exemplary irradianceprofile 96 a where Z<Z₀′, and FIG. 5Bc shows an exemplary irradianceprofile 96 c where Z>Z₀′.

Beam shaping component 90 is preferably a diffractive optic element(DOE) that can perform complex beam shaping with high efficiency andaccuracy. Beam shaping component 90 not only transforms the Gaussianirradiance profile of FIG. 5A to the near-uniform irradiance profile ofFIG. 5Bb, but they also focus the shaped output 94 to a determinable orspecified spot size. Both the shaped irradiance profile 94 b and theprescribed spot size are designed to occur at a design distance Z₀ downstream of beam shaping component 90. In a preferred embodiment, Z₀′ isclose to or equal to distance Z₀. Although a single element DOE ispreferred, skilled persons will appreciate that the DOE may includemultiple separate elements such as the phase plate and transformelements disclosed in U.S. Pat. No. 5,864,430 of Dickey et al., whichalso discloses techniques for designing DOEs for the purpose of beamshaping. Suitable DOEs can be manufactured by MEMS Optical, Inc.,Huntsville, Ala.

FIGS. 6A-6D (collectively FIG. 6) show exemplary substantially uniformirradiance profiles produced by a Gaussian beam propagating through aDOE as described in U.S. Pat. No. 5,864,430. FIGS. 6A-6C show squareirradiance profiles, and FIG. 6D shows a cylindrical irradiance profile.The irradiance profile of FIG. 6C is “inverted,” showing higherintensity at its edges than toward its center. Skilled persons willappreciate that beam shaping components 90 can be designed to supply avariety of other irradiance profiles that might be useful for specificapplications, and these irradiance profiles typically change as afunction of their distance from Z₀′. Skilled persons will appreciatethat a cylindrical irradiance profile such as shown in FIG. 6D ispreferably employed for circular apertures 98; cuboidal irradianceprofiles would be preferred for square apertures; and the properties ofother beam shaping components 90 could be tailored to the shapes ofother apertures. For many straight forward via drilling applications, aninverted cylindrical irradiance profile would be preferred.

With reference again to FIGS. 4-6, shaped pulses 94 are preferablyfocused and passed through an aperture mask 98 to sharpen the edge ofshaped pulses 94. In a preferred embodiment, aperture 98 is positionedat the “nominal aperture plane” which is preferably located at adistance Z from beam shaping component 90 about where Z=Z₀′, Z*, or Z₀.Z* is about the distance that permits a specified desired amount ofenergy of shaped pulse 94 through an aperture 98 of a given desirablediameter d_(ap). Skilled persons will appreciate that in an idealsystem, Z₀=Z₀′=Z*.

While positioning aperture 98 at distance Z₀ would be preferred for mostapplications on a single laser system 50, positioning aperture 98 atdistance Z* is employed for groups of laser systems 50 to address outputvariations from laser 52 to laser 52 and beam shaping element 90 to beamshaping element 90. Z* is preferred because Z* is more sensitive thanZ₀′ such that adjustment within the tolerance of distance Z* will notgenerally deviate the flatness of irradiance profile 96 b to the extentthat it significantly adversely affects via quality or throughput. Anadvantage of using distance Z* for placement of the aperture is that Z*permits a variety of laser systems 50 having variations in Gaussianoutput 54 from lasers 52 to employ the same process parameters fromlaser system 52 to laser system 52 for the same operations. Thus,employing Z* facilitates consistency in documentation, training,synchronization, and via quality.

Mask 98 may comprise a UV reflective or UV absorptive material, but ispreferably made from a dielectric material such as UV grade fused silicaor sapphire coated with a multilayer highly UV reflective coating otherUV resistant coating. Mask 98 has a circular aperture with a diameter ofd_(ap) to produce a highly circular imaged shaped pulses 110. Theaperture of mask 98 may optionally be flared outwardly at its lightexiting side. Skilled persons will appreciate, however, that aperture ofmask 98 can be square, have other noncircular shapes, or even be omittedif images of non-circular spots on the surface of work piece 22 aredesirable or acceptable. Diameter of d_(ap) clips the wings 100 ofshaped pulses 94 to produce an apertured shaped profile 102 thatdecreases the diameter of shaped pulses 94 at the expense of theirtransmitted energy.

The transmitted apertured shaped pulse or output 110 is then collectedby a “first imaging” or collection lens 112 of focal length f₁ toproduce substantially collimated apertured shaped output 72 that ispropagated through positioning system 74 and then re-imaged by scan lens80 of focal length f₂ to produce (targeted apertured shaped) lasersystem pulses or output 114 directed toward work piece 22, and creatingimaged shaped output 118 of spot size diameter d_(spot) on work piece22. In a preferred embodiment, lenses 80 and 112 comprise the imagingoptics useful for significant separation of lenses 80 and 112; however,a skilled person will appreciate that a single imaging lens module orcomponent could be employed. In a preferred embodiment, f₁=216 mm andf₂=81 mm, though persons skilled in the art will recognize that otherlens combinations and focus lengths could be substituted. Thecombination of the collection lens 112 and the scan lens 80 produces animage of the uniformly illuminated aperture of mask 98 (oruniform-irradiance non-circular spot if mask 98 is not used) at ade-magnification factor M, where M=f₁/f₂=d_(ap)/d_(spot). In a preferredembodiment of a fixed de-magnification system, M=2.66, although skilledperson will appreciate that other de-magnification factors could beused.

In a preferred embodiment, beam shaping component 90, aperture mask 98,and first imaging lens 112 are mounted along a rail path on aninterchangeable imaging optics rail (IOR). The rail path is adapted tobe positioned collinearly with the beam path 64. In one embodiment, thedistance Z, f₁, and f₂ are conserved to permit manufacturinginterchangeability of these components in the IOR with analogouscomponents with different properties to perform diverse ranges of spotsizes d_(spot). The positioning of beam shaping component 90 can also bevariable so that distance Z can be adjusted within the tolerances of Z*for each combination of beam shaping components 90 and aperturediameters d_(ap). The effective distance between lenses 112 and 80 isvariable. Thus, several IORs with different combinations of IORcomponents can be quickly exchanged to allow processing operations for adiverse range of predetermined spot sizes. These different combinationsare employed so the beam shape or irradiance profile 96 can be adaptedfor each aperture size d_(ap) to maximize the energy per pulse 62 thatpropagates through the aperture and therefore minimize the energyclipped or wasted by the size limit of the aperture. In addition, toenhancing the efficient use of the pulse energy, the adjustablecoordination between the IOR optical components minimizes any mask 98adaptations that might be desirable to make mask 98 able to withstandlaser damage.

A disadvantage of this embodiment is, however, the large number ofinterchangeable IOR optical components desirable for processing a rangeof useful spot sizes. For example, each beam shaping component 90 may,for example, be efficient for only three to four spot sizes d_(spot),and each mask 98 may, for example, be efficient for only one spot sized_(spot). Thus, to cover the most useful range of spot sizes d_(spot) upto 250 μm, for example, a collection of eight beam shaping components 90and 25 masks 98 might be employed to provide all of the desirablecombinations.

FIGS. 7A-7D are simplified side elevation and partly schematic views offour respective exemplary embodiments of shaped imaging systems 70 a, 70b, 70 c, and 70 d (generically, shaped imaging system 70) for varyingthe size of an imaged spot that include exemplary zoom beam expander(ZBE) lens assemblies 120 a, 120 b, 120 c, and 120 d (generically, ZBEassembly 120). With reference to FIGS. 4 and 7, a ZBE assembly 120 (withtight tolerances to maintain beam accuracy) may be positioned along beampath 64 between aperture mask 98 and work piece 22. ZBE assembly 120preferably includes lenses 124 and 126 that collectively function tovary the spot size. In a preferred embodiment, lens 124 includes a zoomelement and functions to change the magnification of laser system 50 andlens 126 includes a compensator element that functions to collimate thebeam and maintain the focus throughout a zooming operation. Theseembodiments may employ optional alignment mirrors 79 shown in phantom.

In some embodiments, the focal length f₂ is fixed, but the focal lengthf₁ is variable and therefore the de-magnification factor, M, and thespot size d_(spot) are variable, so each beam shaping component 90 mayfor example efficiently accommodate 8-10 spot sizes or a continuouslyvarying spot size within a specific range of sizes, and each aperturemay also efficiently accommodate 8-10 spot sizes or a continuouslyvarying spot size within a specific range of sizes. Thus, to cover therange of spot sizes d_(spot) up to about 250 μm, only a few beam shapingcomponents 90 and masks 98 would be employed. In a preferred embodiment,as few as three combinations of beam shaping components 90 and masks 98could be used to cover the entire range with greater efficiency.

ZBE assembly 120 permits a 1 μm resolution over the continuous range ofdesirable spot sizes and permits very fine tuning, such as forcompensation for different properties of material interactions. Forexample, some materials will exhibit a 50-μm spot after a given laseroperation while other materials will exhibit a 58-μm spot under the sameconditions. ZBE assembly 120 permits, therefore, an easy method formaking consistently-sized vias in different materials.

ZBE assembly 120 can also be employed to change the fluence by changingthe spot size. For example, numerous vias through one material layercould be processed at one spot size and then the spot size could beincreased to lower the fluence for processing vias through all of adifferent material layer.

With reference to FIG. 7A, a zoom lens assembly 120 a (with tighttolerances to maintain beam accuracy) is positioned along beam path 64between first imaging lens 112 and scan lens 80. In zoom lens assembly120 b of the embodiment of shaped imaging system 70 b shown in FIG. 7B,lens 80 and lens 128 are combined into a single lens 130 in zoom lensassembly 120 b. In zoom lens assembly 120 c of the embodiment of shapedimaging system 70 c shown in FIG. 7C, lens 112 and lens 122 are combinedinto a single lens 132 in zoom lens assembly 120 c. In zoom lensassembly 120 d of the embodiment of shaped imaging system 70 d shown inFIG. 7D, lens 80 and lens 128 are combined into a single lens 130 andlens 112 and lens 122 are combined into a single lens 132. Skilledpersons will appreciate that shaped imaging systems 70 a and 70 c arebest suited for a preferred split-axis translation stage positioner 76,and that shaped imaging systems 70 b and 70 d are best suited for beampositioning systems 74 that do not have a fast positioner 78 such asnon-scanning systems that employ a fixed objective lens 80. Skilledpersons will further appreciate that numerous other variable lenscombinations are possible and could be employed without departing fromthe scope of the invention.

Although positioning system 74 is shown following ZBE assembly 120 alongbeam path 64, some of its components may be positioned to precede ZBEassembly 120. For example, some components of translation stagepositioner may be positioned upstream of ZBE assembly 120, such as someof mirrors 75; however, fast positioner 78 is preferably positioneddownstream of ZBE assembly 120. Skilled persons will appreciate thatthese shaped imaging systems 70 may be supported by separate IORs or asingle IOR system that permits exchange and repositioning of the opticalcomponents and that the IORs supporting the shaped imaging systems canbe easily removed to permit laser system 50 to provide Gaussian outputfor versatility.

FIG. 8 is a simplified partly schematic plan view of laser system 150that employs galvanometers 152 and 154 to produce an auxiliarygalvanometer mirror pathway 156 that can be added to laser system 50 ofFIG. 4 to permit switching between collimated apertured shaped output 72and Gaussian output 60. With reference to FIG. 8, beam path 64 a isdirected toward galvanometer mirror 158 that either permits the laseroutput to propagate along beam path 64 b through shaped imaging system70 and by galvanometer mirror 162 or reflects the laser output offmirror 164, through optional collimating lens components 166, off mirror168, off galvanometer mirror 162, and toward work piece 22. Mirrors 164and 168 can preferably be adjusted to compensate for pitch and roll.

Skilled persons will appreciate that collimating lens components 166 canbe variable to modify the spatial spot size d_(spot) to suit differentapplications. Alternatively, for example, shaped imaging system 70 caninstead be positioned along pathway 156 to implement collimatedapertured shaped output 72 so the raw Gaussian beam 60 would propagatealong beam path 64 b. Similarly, a shaped imaging system 70 can beemployed in both beam path 64 b and pathway 156 with each shaped imagingsystem 70 having variable or different focal lengths to producedifferent spot sizes d_(spot), such as for quick switching between twodifferent imaged shaped spot sizes. Skilled persons will also appreciatethat laser system 150 could employ the Gaussian output to perform avariety of tasks in addition to the via processing applicationsdiscussed herein. For example, laser system 150 could be used to cutcircuits out of panels at high throughput rates.

Laser systems 50 and 150 are capable of producing laser system havingoutput 114 having preferred parameters of typical via processing windowsthat may include average power densities greater than about 100 mWmeasured over the beam spot area, and preferably greater than 300 mW;spot size diameters or spatial major axes of about 5 μm to about 18 μm,preferably from about 25-150 μm, or greater than 300 μm; and arepetition rate of greater than about 1 kHz, preferably greater thanabout 5 kHz or even higher than 30 kHz; an ultraviolet wavelength,preferably between about 180-355 nm; and temporal pulse widths that areshorter than about 100 ns, and preferably from about 40-90 ns orshorter. The preferred parameters of laser system output 114 areselected in an attempt to circumvent thermal damage to via 20 or itssurroundings. Skilled persons will also appreciate that the spot area oflaser system output 114 is preferably circular, but other simple shapessuch as squares and rectangles may be useful and even complex beamshapes are possible with the proper selection of beam shaping component90 cooperating with a desirable aperture shape in mask 98.

The above-described processing window has been determined to facilitatevia drilling in a wide variety of metallic, dielectric, and other targetmaterials that exhibit diverse optical absorption and othercharacteristics in response to ultraviolet light. Whether punching ornonpunching to create blind vias 20 b, the metal layer is removed with afirst laser output having a power density sufficient to ablate themetal. Then, the dielectric layer is removed with a second laser outputhaving a lower power density that is insufficient to ablate the metal,so only the dielectric is removed and the underlying metallic layer isnot damaged. Thus, the two-step machining method provides a depthwiseself-limiting blind via because the second laser power output isinsufficient to vaporize the metallic bottom layer, even if the secondlaser power output continues after the dielectric material is completelypenetrated.

Skilled persons will appreciate that in accordance with a punchingprocess of the present invention, the first and second laser outputs arepreferably sequentially contiguous rather than employing a series offirst laser outputs one at a time to spatially separated targetpositions 82 or work piece 22 and then employing a series of secondlaser outputs sequentially over the same targets 82. For a nonpunchingprocess, layers 24 of all of the spatially separated target positions 82on work piece 22 may be processed with the first laser outputs beforethe layers 26 of all of the spatially separated target positions 82.

With reference again to FIGS. 3 and 4, one difference between theclipped Gaussian output of the prior art and imaged shaped output 118 ofthe present invention is that pulse 94 uniformly illuminates theaperture of mask 98 at all points. The imaged shaped output 118consequently facilitates formation of blind vias 20 b with a very flatand uniform bottom 44 b at layer 28 in addition to a very round shapeand crisp edges, and this flatness and uniformity are not possible witha clipped Gaussian beam. In addition, the drilling speed can beincreased with imaged shaped output 118 over that obtainable with aclipped Gaussian beam.

The addition of a beam shaping component 90 to flatten the irradianceprofile 10 of a Gaussian beam minimizes the previously discussedprocessing tradeoffs between via quality and drilling speed inherent tothe clipped Gaussian technique. A high fraction of the beam energy canbe delivered to work piece 22 without a large difference in fluencebetween the center and edges of the imaged spot, i.e. the edge-to-centerfluence ratio Fe/Fc can be increased while transmission level T is alsoincreased. The present invention permits apertured shaped output 110 andimaged shaped output 118 to have transmission levels of 70-85% without asignificant decrease in center to edge fluence ratio.

As a result of the near-uniform fluence at high transmission levels, thedrilling speed can be increased without damaging conductor layer 28,particularly at its center, for two reasons. First, the transmissionlevel through the aperture is higher than for the clipped Gaussian, somore energy is delivered to the work piece 22 in each laser pulse 114.Second, since a higher fluence can be applied to the edges of the spot,the dielectric material can be cleared from the bottom edges of the viamore rapidly. This second effect is the more significant of the two.

FIG. 9 shows a comparison of ideal fluence profiles at the apertureplane for shaped output 94 b and clipped Gaussian output at severaltypical transmission levels under typical via processing parameters.Fluence levels on the work piece 22 are equal to the aperture fluencelevels multiplied by the imaging de-magnification factor squared, whichin a preferred embodiment is about a factor of seven. The fluences atthe aperture edge are about 1.05 J/cm² and 0.60 J/cm² or less for shapedoutput 94 b and clipped Gaussian output, respectively. Thus, at workpiece 22, the fluences at the edge of the imaged spot are about 7.4 and4.3 J/cm² for the imaged shaped output 118 and clipped Gaussian output,respectively. The rate at which a typical organic dielectric material oflayer 26 can be ablated differs significantly between these two fluencelevels. As a result, drilling of each via 20 can be completed in fewerpulses with the imaged shaped output 118, increasing the processthroughput.

An example of a strategy for drilling vias 20 with imaged shaped output118 in accordance with these considerations of present invention isdescribed below. The fluence across the entire imaged spot can bemaintained, for example, at 90% of the value at which copper damageoccurs, F_(damage). The dielectric material is then ablated atconditions which will not damage via bottom 44 b. In contrast, with theclipped Gaussian beam at T=50%, one could maintain the center of thespot at this fluence, in which case the edges would be at only 45% ofF_(damage). Alternatively, the spot edge could be held at 90% ofF_(damage), in which case the center would be at 180% of the damagethreshold fluence, resulting in substantial damage. Maintaining theedges of the imaged spot at high fluence enables the dielectric materialto be cleared from the via edges with fewer laser pulses, since eachpulse removes more material. Thus, the drilling throughput of imagedshaped output 118 can be much greater than that of the clipped Gaussianoutput.

FIG. 10 shows a comparison of the throughput curves achieved by theimaged shaped output 118 and the clipped Gaussian output for punching 75μm-diameter vias 20 in 45 μm-deep epoxy resin. With reference to FIG.10, the minimum number of pulses, N, necessary to achieve a bottomdiameter d_(b) at least 75% as large as the top diameter d_(t) at eachpulse repetition frequency (PRF) was determined. The drilling time wascalculated for this value of N at the PRF, and a 1.0 ms via-to-via movetime was added to obtain the throughput.

In general, as the laser PRF increases, the energy in each pulse, andtherefore the work-surface fluence, steadily decreases. Since decreasedfluence means less material is removed per pulse, more pulses must beapplied. However, as the PRF increases, more pulses are delivered perunit time. The net result is that of two competing effects, one of whichtends to decrease drilling speed and the other of which tends toincrease drilling speed with increasing PRF. FIG. 10 shows that thecompeting effects yield the fastest throughputs at PRFs in the middle ofthe range tested.

FIG. 10 also shows that the throughput curve achieved with imaged shapedoutput 118 is flatter than that obtained with clipped Gaussian output.The flatter throughput curve is significant for managing the tradeoffbetween drilling speed and via quality. In order to avoid damage to thebottom metallic layer 28, it is generally desirable to increase thelaser PRF, thereby decreasing the energy in each pulse and reducing thework-surface fluence below the energy threshold for melting metalliclayer 28. As the PRF is increased, the throughput obtained with theimaged shaped output 118 decreases more slowly than that of the clippedGaussian output. So as the PRF is increased in order maintain via bottomquality, less of a throughput penalty is incurred with the imaged shapedoutput 118.

With reference again to FIG. 10, the imaged shaped output 118 enablesthe peak drilling throughput to be increased over that of the clippedGaussian by more than 25%. The imaged shaped output 118 also achieveshigher throughput than is achieved with a raw focused Gaussian beam,with the added benefits of better via quality (repeatability, sidewalltaper, roundness).

With respect to via quality, particularly for blind vias 20 b, theimaged shaped output 118 of the present invention also permits bettertaper minimizing performance at higher throughput rates than thatavailable with clipped Gaussian output. In addition to being able toclean dielectric material from the bottom edges of blind via 20 b fasteras discussed above, the imaged shaped output can also clean thedielectric material from the bottom edges of via 20 b more completelywithout risking damage to the underlying metallic layer 28 because theuniform shape of pulse 94 virtually eliminates the possibility ofcreating a hot spot in layer 28 at the bottom center of via 20 b. Theimaged shaped laser output 118 at an appropriate fluence can dwell in ablind via hole indefinitely until a desired cleanliness and taper isachieved.

Moreover, beam shaping components 90 can be selected to produce pulseshaving an inverted irradiance profile shown in FIG. 6C that is clippedoutside dashed lines 180 to facilitate dielectric removal along theouter edges of via 20 b and thereby further improve taper. The presentinvention permits a taper ratio of greater than 80% at a maximumthroughput with no damage to layer 28, and taper ratios of greater than95% (for low aspect ratio vias 20) are possible without damage to layer28. Better than 75% taper ratios are even possible for the smallestvias, from about 5-18 μm diameter at the via top with conventionaloptics although throughput might be diminished.

FIG. 11 shows the ratio of via bottom diameter to the via top diameter(62 μm vias drilled in 35 μm particulate-reinforced epoxy resin) as afunction work surface location relative to the nominal image plane, z=0.With reference to FIG. 11, the nominal image plane is the location wherethe vias 20 are most circular, with the most sharply defined top edges.Positive values of z represent planes below the nominal image plane,i.e., with the work piece 22 placed farther from the system optics thandistance of separation where z=0. The 3σ error bar is shown forreference because bottom diameter measurements may be difficult tomeasure reliably.

One hundred vias were drilled and measured at each of nine values of z.The data points represent mean values and the vertical error barsrepresent the magnitude of three standard deviations from the mean overeach 100-sample data set. The largest bottom/top ratio is achieved atthe image plane where z=0. Throughout a ±400 μm range, the bottom/topratio was always greater than 75% at high throughput.

FIG. 12 shows via diameter (in 62-μm vias drilled in 35 μm-particulatereinforced epoxy resin) as a function of work surface location relativeto the nominal image plane, where z=0. As the work piece 22 is movedfurther above the nominal image plane, the average via top diameterincreases steadily. For locations below z=0, the top diameter remainsfairly constant out to 400 μm below the image plane. The 3σ diametersare generally held to within ±3 μm of the average value, with exceptionsat z=+300 μm and z=−300 μm. For the bottom diameters, in contrast, theaverage value decreases steadily from locations above to locations belowthe nominal image plane. Because the diameter and circularity of the viabottom are significantly more difficult to control than the size androundness of the via top, the bottom diameter is shown for referenceonly. Statistical process control techniques that could be applied tolaser systems 50 and 150 are, therefore, applicable to thecharacteristics of the via tops.

The data in FIGS. 11 and 12 suggest several approaches to managing depthof focus issues for process robustness. If one wishes to maintain aconstant via top diameter over varying material thicknesses and machineconditions, it would be advantageous to set up the process with the worksurface located slightly below the nominal image plane at, say z=+200μm. This would produce a zone of ±200 μm of z variation that could beaccommodated with very little effect on the top diameter. If, on theother hand, it is more desirable to maintain a constant via bottom/topdiameter ratio, it would be better to set up the process with work piece22 located exactly at the nominal image plane. This would ensure thatthe bottom/top ratio would decrease by no more than 5% over a z range ofat least ±200 μm. The viability of either of these approaches depends onwhether the other via characteristics remain within acceptable limits aswork piece 22 moves away from the nominal image plane.

Another issue is via circularity, which is shown in FIG. 13 as afunction of z for 62-μm vias drilled in 35-μm particulate-reinforcedepoxy resin. With reference to FIG. 13, the bottom circularity data havebeen displaced to the right of the actual z values for clarity ofpresentation. The via bottom data are for reference only.

FIG. 13 shows that the circularity, defined as the minor axis/majoraxis, is always at least 90% over the full ±400 μm z range of the study.For a 62-μm average diameter, 90% circularity corresponds to a majordiameter that is about 6.5 μm larger than the minor diameter. However,for positive z values (locations below the nominal image plane), thestatistical via-to-via variation in circularity become appreciable. Theerror bars shown above the data points (average values) are meaninglessabove 100% circularity, but at, for example, z=+300 μm, FIG. 13 showsthat the 3σ outliers may have circularity below 80%.

In general, the imaged shaped output 118 of present invention permitsvias 20 to have a roundness or circularity of greater than 90% at higherthroughput rates than achievable with clipped Gaussian output. In manycases, imaged shaped output 118 can achieve a roundness of greater thaneven 95% over the entire range of via sizes at higher throughput rates.

Although some of the examples described herein address some of themaximum output and other factors involving the use of currentlyavailable UV DPSS lasers 52, skilled persons will appreciate that asmore powerful UV DPSS lasers 52 become available, the via diameters andlayer thicknesses in these examples can be increased.

Despite the advantages of imaged shaped output 118, projection imagingmay spread the available energy in each imaged shaped laser pulse 118over a larger area than that typically covered by the ablative portionof a focused raw Gaussian beam. As a result, UV DPSS lasers 52 haveenergy per pulse limits to the size and thickness of metallic layers 24and 28 where the laser spot will exceed the ablation threshold fluencefor the work piece materials.

With respect to blind vias, for example, imaged shaped pulses 118 withfluences of 10-12 J/cm² may be employed to ablate a top copper layer 24of 5-12 μm thick for small vias up to perhaps 40 μm in diameter. Skilledpersons will appreciate that this fluence range implies a fairly slowrepetition rate of about 3-6 kHz, for example. Skilled persons will alsoappreciate that higher fluences may invite adverse consequences such asheating, and the resulting slower repetition rate would negativelyimpact throughput. As the power obtainable with UV DPSS lasers 52continues to increase, higher-energy pulses will be available which willextend the shaped imaging technique to through-copper applications forlarger via sizes.

In the interim, a preferred method for punching the top metallic layer24 of blind vias 20 b having diameters greater than about 35 μm employslaser system 150 of FIG. 8. The galvanometer mirror pathway 156 isemployed to provide focused raw Gaussian output as laser system output114. The focused raw Gaussian output is used to penetrate the topmetallic layer 24, typically using a nonpunching technique, and then thegalvanometer mirrors 158 and 162 are controlled to allow the laseroutput 60 to pass through imaging system 70 for processing dielectriclayer 26. Skilled persons will appreciate that other types of faststeering mirror methods are well known and could be employed.

Regardless of how top metallic layer 24 is processed (or it may even bepre-etched), the underlying dielectric layer 26 can subsequently bemachined with imaged shaped output 118 with lower fluences at higherrepetition rates to produce vias 20 with clean round bottoms andnegligible taper as previously described. Typical dielectric processingfluences range from below about 0.8 J/cm², which does little to nodamage to bottom metallic layer 28, to above about 4 J/cm², whichimparts substantial damage to bottom metallic layer 28. Although thepreferred fluence is material dependent, fluences of 1.2-1.8 J/cm² arepreferred for most dielectric layers 26 as the imaged shaped pulses 118approach a copper metallic layer 28.

Skilled persons will appreciate that there is a throughput advantage toprocessing the upper portion of dielectric layer 24 at a fluence at thehigher end of this range and then reducing the fluence (preferably byincreasing the repetition rate) toward the lower end of the range as thelaser pulses 114 get close to bottom metallic layer 28. For optimumthroughput, repetition rates of 12-45 kHz are preferred, 12-15 kHz forlarger vias 20 b and hard to ablate layers 26 and 30-45 kHz for smallervias. Skilled persons will appreciate that these repetition rate valueswill increase as available DPSS laser power improves in the future.

In some applications for medium sized blind vias 20 b, it may bedesirable to use the fast positioner 78 to process the top metalliclayer 24 by nonpunching with the focused Gaussian output and thenpunching through the dielectric with imaged shaped output 118. Skilledpersons will also appreciate that the focused raw Gaussian output oflaser system 150 can also be employed for processing through-hole vias20 a where the via diameters are too large for efficient imaged shapedoutput 118 or where speed is more important than roundness or edgequality.

With respect to processing the organic or inorganic dielectric materialsof layer 26, they typically have a much lower ablation threshold and areeasily ablated with a projection imaging configuration up to the largestdesirable via diameters. However, for larger via sizes of about 150 μmto about 200 μm and larger, depending on the properties of theparticular material, the energy distribution of imaged shaped output 118over the via diameter diminishes to a point where the throughput isadversely affected because each laser system pulse 114 removes lessmaterial.

In applications where via diameters exceed about 250-300 μm in size andedge quality and perfect roundness are not as important as throughput,imaged shaped output 118 or focused Gaussian output of laser system 150is preferably employed to create via 20 by nonpunch processing employingfast positioner 78. Skilled persons will appreciate that nonpunchprocessing can produce acceptable taper and roundness for large vias 20to suit most applications. This preference applies to both through-holeand blind via processing. Skilled persons will also appreciate that theimaged shaped output 118 may be more efficient than the focused Gaussianoutput for large size vias in many applications.

FIG. 14 shows a scanning electron micrograph (SEM) of a typical viadrilled in epoxy resin with the imaged shaped output 118 of a UV YAGlaser system 50. The via diameter was 75 μm, the resin thickness was 45μm, and the substrate was prepared by etching away the top copper layerof a resin coated foil on an FR4 core. The bottom (or inner) layercopper was 18 μm (½ oz).

Several features are noteworthy. First, the via sidewalls areexceptionally smooth and straight, and the top edge of the via issharply defined. Second, owing to image projection of the roundaperture, the via is particularly circular, as previously described.Finally, the bottom copper layer is largely undamaged and free of anyresin residue.

For this particular test, the beam shaping optics were configured toproduce an inverted fluence profile (FIG. 6C) at the work surface thatwas slightly higher at the spot periphery than in the center. The laserparameters (PRF and number of pulses applied) were then adjusted toproduce a work-surface fluence at the periphery that was just above thevalue that induces melting of the copper. Close inspection of the imagereveals that the smooth areas near the edges of the via bottom areregions where the copper was lightly reflowed. Such light reflowing ofthe copper may be desired in order to ensure that all resin has beenremoved from the via. This degree of control of the inner layer copperdamage is typical of vias 20 produced by imaged shaped output 118.

For HDI circuit board microvias, the most common laser drillingtechnique in resin coated foil constructions makes use of a circularopening pre-etched in the top copper layer. This opening is used as aconformal mask for CO₂ laser processing. Layer-to-layer registrationdifficulty has limited this process to drilling of larger vias (>100 μm)with larger pad sizes (>200 μm). However, positioning system 74 permitsthe accurate layer-to-layer alignment of a laser drilling system to becoupled with the higher throughput of drilling only the dielectricresin. In this new process, the outer layer copper is pre-etched to theapproximate size of the inner layer land pad, and the laser is then usedto align and drill a smaller via within this opening. A scanningelectron micrograph of an exemplary via 20 that could be created by thisprocess is presented in FIG. 15, which shows an imaged shaped 75-μm viathrough 45-μm epoxy resin in a 150-μm pre-etched copper opening.

Skilled persons will appreciate that the beam shaping and imagingtechniques described herein not only permit enhanced via roundness andedge quality, but also permit enhanced repeatability and positioningaccuracy such as in the exact center of pads and may be useful forimproving impedance control and predictability of the electronic workpieces.

Further comparative data between shaped imaging and clipped Gaussiantechniques, including color electron micrographs, can be found in thearticle entitled “High Quality Microvia Formation with Imaged UV YAGLasers,” which was presented as a portion of the Technical Proceedingsof the IPC Printed Circuits Expo 2000 in San Diego, Calif. on Apr. 6,2000.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments of thisinvention without departing from the underlying principles thereof. Thescope of the present invention should, therefore, be determined only bythe following claims.

What is claimed is:
 1. A laser system, comprising: a diode-pumped,solid-state laser for generating ultraviolet Gaussian laser outputhaving a Gaussian energy along an optical path; a beam-shaping elementpositioned along the optical path for converting the Gaussian laseroutput to shaped output having a central irradiance profile of high anduniform intensity and an outer irradiance profile of low intensity; avariable beam expansion assembly positioned along the optical pathbetween the laser and the beam-shaping element; an aperture positionedalong the optical path for clipping a major portion of the outerirradiance profile of the shaped output and passing at least 50% of theGaussian energy through the aperture to produce apertured shaped outputhaving apertured shaped energy; one or more imaging lens components forconverting the apertured shaped output into image shaped output; and apositioning system for directing the imaged shaped output toward atarget location on a work piece.
 2. The laser system of claim 1 in whichthe beam-shaping element comprises a diffractive optical element.
 3. Thelaser system of claim 1 in which the laser system forms one of a bank ofat least two substantially similar laser systems for performing the sameprocessing application, the laser systems having respective lasers andrespective diffractive optical elements with unintentional performancedifferences, and in which the variable beam expansion assembly of atleast one of the laser systems to compensate for an unintentionalperformance difference between the lasers or between the diffractiveoptical elements.
 4. The laser system of claim 1 further comprising azoom lens assembly positioned along the optical path between theaperture and a target location on a work piece.
 5. The laser system ofclaim 1 in which the apertured shaped energy is greater than 65% of theGaussian energy.
 6. The laser system of claim 5 in which the aperturedshaped energy is greater than 75% of the Gaussian energy.
 7. The lasersystem of claim 1 in which the wavelength is about 355 nm or about 266nm.
 8. The laser system of claim 1 in which the diffractive opticalelement is a first diffractive optical element, the aperture is a firstaperture having a first size, and the first diffractive optical elementand the first aperture cooperate to produce a substantially uniformfirst energy density over a first spot area on the work piece.
 9. Thelaser system of claim 8 in which the first diffractive optical elementand the first aperture are removable and replaceable by a seconddiffractive optical element and a second aperture which cooperate todetermine a second substantially uniform energy density over a secondspot area, which is different from the first spot area, on the workpiece.
 10. The laser system of claim 8, further comprising: a firstremovable imaging optics rail that houses the first diffractive opticalelement and the first aperture, the first imaging optics rail beingreplaceable with a second imaging optics rail having a seconddiffractive optical element and a second aperture which cooperate todetermine a second energy density over a second spot area, which isdifferent from the first spot area, at the target location on the workpiece.
 11. The laser system of claim 1, further comprising: a removableimaging optics rail that houses the diffractive optical element and theaperture, such that removal of the imaging optics rail permits theGaussian laser output to impinge the work piece at the target location.12. A laser system, comprising: a diode-pumped, solid-state laser forgenerating ultraviolet Gaussian laser output having a Gaussian energyalong an optical path; a beam-shaping element positioned along theoptical path for converting the Gaussian laser output to shaped outputhaving a central irradiance profile of high and uniform intensity and anouter irradiance profile of low intensity; an aperture positioned alongthe optical path for clipping a major portion of the outer irradianceprofile of the shaped output and passing a major portion of the Gaussianenergy through the aperture to produce apertured shaped output havingapertured shaped energy; a zoom lens assembly positioned along theoptical path between the aperture and a target location on a work piece;one or more imaging lens components for converting the apertured shapedoutput into imaged shaped output; and a positioning system for directingthe imaged shaped output toward the target location to form a via. 13.The laser system of claim 12 in which the imaged shaped output has aspot size at the target in a spot size range between about 10 and 250 μmand in which the zoom lens assembly facilitates spot sizes substantiallythroughout the spot size range with resolution of about 1 μm.
 14. Thelaser system of claim 12 in which first and second target materials havedifferent laser ablation characteristics such that a given set of laserparameters would form a first via in the first target material of afirst major spatial dimension and the same set of laser parameters wouldform a second via in the second target material of a second majorspatial dimension and such that manipulation of the zoom lens assemblyfacilitates the formation of a third via in the second target materialof the first major spatial dimension with a substantially similar set oflaser parameters.
 15. The laser system of claim 12 in which thewavelength is about 355 nm or about 266 nm.
 16. The laser system ofclaim 12 in which the target location comprises a metal material layerand a dielectric material layer and the zoom lens assembly provides asmaller spot size for processing the metal material layer and a largerspot size for processing the dielectric material layer.
 17. The lasersystem of claim 12 in which the dielectric material layer comprises anorganic dielectric material and the metal material layer comprisescopper.
 18. The laser system of claim 12 in which the central irradianceprofile has a profile size when it reaches the aperture, the aperturehas an aperture size that influences the percentage of Gaussian energythat reaches the target, and the zoom lens assembly is adjusted to matchsubstantially the size of the profile size with the aperture size.
 19. Alaser system, comprising: a diode-pumped, solid-state laser forgenerating Gaussian laser output having a Gaussian energy along anoptical path; a first removable imaging optics rail positioned along theoptical path, the first imaging optics rail housing a first beam shapingcomponent and a first aperture, the first beam shaping component beingpositioned along a first rail path collinear with the optical path toconvert the Gaussian laser output to first shaped output having a firstcentral irradiance profile of high and uniform intensity and a firstouter irradiance profile of low intensity, the first aperture beingpositioned along the optical path to clip a first portion of the firstouter irradiance profile of the first shaped output and pass at least50% of the Gaussian energy through the first aperture to produce firstapertured shaped output having first apertured shaped energy, the firstimaging optics rail being replaceable with a second imaging optics railhousing a second beam shaping component and a second aperture, thesecond beam shaping component being positioned along a second rail pathdesigned to be collinear with the optical path to convert the Gaussianlaser output to second shaped output having a second central irradianceprofile of high and uniform intensity and a second outer irradianceprofile of low intensity, the second aperture being positioned along theoptical path to clip a second portion of the second outer irradianceprofile of the second shaped output and pass at least 50% of theGaussian energy through the second aperture to produce second aperturedshaped output having second apertured shaped energy, wherein the secondapertured shaped energy is of an amount different from that of the firstapertured shaped energy; one or more imaging lens components forconverting the first or second apertured shaped output into respectivefirst or second image shaped output, wherein the first image shapedoutput has a first irradiance on a work piece and the second imagedshaped output has a second irradiance on the work piece different fromthe that of the first irradiance profile; and a positioning system fordirecting the imaged shaped output toward a target location on the workpiece.
 20. The laser system of claim 19, further comprising: a pair ofbeam directing mirrors positioned along the optical path, thediffractive optical element and the aperture being positioned opticallybetween the beam directing mirrors, for diverting the Gaussian laseroutput along an alternative optical path that avoids the diffractiveoptical element and the aperture such that the beam positioning systemdirects the Gaussian output toward the work piece.
 21. The laser systemof claim 19 in which the diffractive optical element is a firstdiffractive optical element, the aperture is a first aperture having afirst size, and the first diffractive optical element and the firstaperture cooperate to produce a substantially uniform first energydensity over a first spot area on the work piece.
 22. The laser systemof claim 21 in which the first diffractive optical element and the firstaperture are removable and replaceable by a second diffractive opticalelement and a second aperture which cooperate to determine a secondsubstantially uniform energy density over a second spot area on the workpiece.
 23. The laser system of claim 21, further comprising: a firstremovable imaging optics rail that houses the first diffractive opticalelement and the first aperture, the first imaging optics rail beingreplaceable with a second imaging optics rail having a seconddiffractive optical element and a second aperture which cooperate todetermine a second energy density over a second spot area at the targetlocation on the work piece.
 24. The laser system of claim 19, furthercomprising: a removable imaging optics rail that houses the diffractiveoptical element and the aperture, such that removal of the imagingoptics rail permits the Gaussian laser output to impinge the work pieceat the target location.
 25. The laser system of claim 19 in which theapertured shaped energy is greater than 65% of the Gaussian energy. 26.The laser system of claim 25 in which the apertured shaped energy isgreater than 75% of the Gaussian energy.
 27. The laser system of claim19 in which the via has a minimum diameter d_(min), a maximum diameterd_(max), and a roundness of greater than 0.9, where the roundness equalsd_(min)/d_(max).
 28. The laser system of claim 19 in which the via has abottom diameter d_(b), a top diameter d_(t), and a taper ratio ofgreater than 0.5, where the taper ratio equals d_(b)/d_(t).
 29. Thelaser system of claim 19 in which the first energy density is less thanor equal to about 2 J/cm².
 30. The laser system of claim 19 in which thewavelength is about 355 nm or about 266 nm.
 31. The laser system ofclaim 19 in which the via comprises first and second layer materials,and the first layer material comprises a dielectric material and thesecond layer material comprises a metal.
 32. The laser system of claim31 in which the dielectric material comprises an organic dielectricmaterial and the metal comprises copper.
 33. The laser system of claim19 further comprising a variable beam expander positioned along theoptical path between the aperture and the work piece.
 34. The lasersystem of claim 19 further comprising rapidly variable controlelectronics for rapidly changing control of a Q-switch for effecting arapid repetition rate change in the Gaussian laser output to convert itto a second Gaussian laser output having a second pulse energy.
 35. Amethod for substantially matching the performance of substantiallysimilar first and second laser systems, the first laser system having afirst diode-pumped, solid-state laser for generating first ultravioletGaussian laser output having a first Gaussian energy along a firstoptical path; a first diffractive optical element positioned along thefirst optical path for converting the first Gaussian laser output tofirst shaped output having a first central irradiance profile of highand uniform intensity and a first outer irradiance profile of lowintensity, a first variable beam expansion assembly positioned along thefirst optical path between the first laser and the first diffractiveoptical element, a first aperture positioned along the first opticalpath for clipping a major portion of the first outer irradiance profileof the first shaped output and passing at least 50% of the firstGaussian energy through the first aperture to produce first aperturedshaped output having first apertured shaped energy, one or more firstimaging lens components for converting the first apertured shaped outputinto first image shaped output; and a first positioning system fordirecting the first imaged shaped output toward a first target locationon a first work piece, the second laser system having a seconddiode-pumped, solid-state laser for generating second ultravioletGaussian laser output having a second Gaussian energy along a secondoptical path; a second diffractive optical element positioned along thesecond optical path for converting the second Gaussian laser output tosecond shaped output having a second central irradiance profile of highand uniform intensity and a second outer irradiance profile of lowintensity, a second variable beam expansion assembly positioned alongthe second optical path between the second laser and the seconddiffractive optical element, a second aperture positioned along thesecond optical path for clipping a major portion of the second outerirradiance profile of the second shaped output and passing at least 50%of the second Gaussian energy through the second aperture to producesecond apertured shaped output having second apertured shaped energy,one or more second imaging lens components for converting the secondapertured shaped output into second image shaped output; and a secondpositioning system for directing the second imaged shaped output towarda second target location on a second work piece, the first and secondlasers or the first and second diffractive optical elements haveunintentional performance differences, comprising: impinging the firsttarget with the first imaged shaped output of the first laser system;impinging the second target with the second imaged shaped output of thesecond laser system; comparing first results of impingement with thefirst imaged shaped output to second results of impingement with thesecond imaged shaped output; and employing the first and/or secondvariable beam expansion assemblies to compensate for performancedifferences between the first and second results such that the first andsecond laser systems exhibit substantially the same performance for agiven laser application.
 36. A method for substantially matching asecond performance of a laser system on a second target of a secondmaterial to a first performance of the laser system on a first target ofa first material, the laser system including a diode-pumped, solid-statelaser for generating ultraviolet Gaussian laser output having a Gaussianenergy along an optical path, a beam-shaping element positioned alongthe optical path for converting the Gaussian laser output to shapedoutput having a central irradiance profile of high and uniform intensityand an outer irradiance profile of low intensity, an aperture positionedalong the optical path for clipping a major portion of the outerirradiance profile of the shaped output and passing a major portion ofthe Gaussian energy through the aperture to produce apertured shapedoutput having apertured shaped energy, a zoom lens assembly positionedalong the optical path between the aperture and a target location on awork piece, one or more imaging lens components for converting theapertured shaped output into image shaped output, and a positioningsystem for directing the imaged shaped output toward the target locationto form a via, the first and second target materials having differentlaser ablation characteristics such that a given set of laser parameterswould form with a first imaged shaped output a first via of a firstmajor spatial dimension in the first target material and the same set oflaser parameters would form with a second imaged shaped output a secondvia of second major spatial dimension in the second target material,comprising: impinging the first target material with the first imagedshaped output of the first spot size to form the first via of the firstmajor spatial dimension; impinging the second target with the secondimaged shaped output of the second spot size to form the second via ofthe second major spatial dimension; comparing the first and secondspatial dimensions; and employing the zoom lens assembly to compensatefor differences between the first and second major spatial dimensionssuch that vias formed in the first and second target materials exhibitsubstantially similar major spatial dimensions with substantiallysimilar laser parameters.